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Date: Mon, 13 Sep 2021 10:21:21 -0600 From: Peter Gonda <pgonda@...gle.com> To: Sean Christopherson <seanjc@...gle.com> Cc: kvm list <kvm@...r.kernel.org>, Marc Orr <marcorr@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, David Rientjes <rientjes@...gle.com>, "Dr . David Alan Gilbert" <dgilbert@...hat.com>, Brijesh Singh <brijesh.singh@....com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH 1/3 V7] KVM, SEV: Add support for SEV intra host migration On Thu, Sep 9, 2021 at 7:12 PM Sean Christopherson <seanjc@...gle.com> wrote: > > On Fri, Sep 10, 2021, Sean Christopherson wrote: > > Ooh, this brings up a potential shortcoming of requiring @dst to be SEV-enabled. > > If every SEV{-ES} ASID is allocated, then there won't be an available ASID to > > (temporarily) allocate for the intra-host migration. But that temporary ASID > > isn't actually necessary, i.e. there's no reason intra-host migration should fail > > if all ASIDs are in-use. Ack forcing dst to be SEV disabled will mitigate this problem. > > ... > > > So I think the only option is to take vcpu->mutex for all vCPUs in both @src and > > @dst. Adding that after acquiring kvm->lock in svm_sev_lock_for_migration() > > should Just Work. Unless userspace is misbehaving, the lock won't be contended > > since all vCPUs need to be quiesced, though it's probably worth using the > > mutex_lock_killable() variant just to be safe. > > Circling back to this after looking at the SEV-ES support, I think the vCPUs in > the source VM need to be reset via kvm_vcpu_reset(vcpu, false). I doubt there's > a use case for actually doing anything with the vCPU, but leaving it runnable > without purging state makes me nervous. > > Alternative #1 would be to mark vCPUs as dead in some way so as to prevent doing > anything useful with the vCPU. > > Alternative #2 would be to "kill" the source VM by setting kvm->vm_bugged to > prevent all ioctls(). > > The downside to preventing future ioctls() is that this would need to be the > very last step of migration. Not sure if that's problematic? I'll add calls to kvm_vcpu_reset. Alternative #2 using vm_bugged won't work for us because we need to keep using the source VM even after the state is transfered.
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